Electro-osmotic pumps operate on the principle that the application of an electric field across a pumping medium, in the presence of a liquid may cause the bulk of the liquid to flow through the pumping medium. This is based on the principle electro-osmotic flow. For the case of water in contact with silicon dioxide or glass, the solid surface may acquire a finite charge density known as an electrical double layer when in contact with the aqueous solution through the deprotonation of silanol groups. As a charge is applied across the pumping medium, the ions will flow from the anode to the cathode and drag the bulk of the aqueous solution with it, creating a positive flow.
Recently, electro-osmotic pumps have been proposed for use with microelectronic devices. For instance, published U.S. patent application Ser. No. 10/053,859 to Goodson, et al., Publication No. 2003/0062149, published on Apr. 3, 2003 describes using electro-osmotic pumps for thermal regulators for microelectronics devices. The electro-osmotic pump that may be used with microelectronic devices that are capable of generating high pressure and flow without moving mechanical parts and the associated generation of unacceptable electrical and acoustical noise.
U.S. Patent Application Publication No. 2003/0147225 to Thomas William Kenny, Jr. et al., describes a method for integrating thermal management of microelectronic devices within the microelectronic device. Therefore, instead of being an “add-on” device, the electro-osmotic pump may be integrated within the microelectronic device.
Published U.S. patent application Ser. No. 10/272,048 to Juan G. Santiago et al., Publication No. 2003/0085024, published on May 8, 2003 describes a method for removing excess gases from closed loop electro-osmotic pumps. The method includes using a gas permeable membrane, which removes and vents electrolytic gases generated by the fluid chamber within the electro-osmotic pump. A catalyst may be used to recombine the electrolytic gases to form a vapor product that may be vented or condensed back to a liquid. The condensed electrolytic vapors may then be passed through an osmotic membrane back to the fluid chamber.
Published U.S. patent application Ser. No. 10/384,000 to Thomas William Kenny Jr. et al., Publication No. 2003/0173942, published on describes an apparatus that integrates the power management module and a thermal management module, such as an electro-osmotic pump, may then be affixed directly to a power consuming microelectronic device.
Published U.S. patent application Ser. No. 10/385,086 to Kenneth E. Goodson et al., Publication No. 2003/0164231, published on Sep. 4, 2003, describes an apparatus for controlling the thermal management of a microelectronic device through electrically controlling the flow of cooling liquid through the pump to minimize the spatial and temporal temperature variations that may occur on the microelectronic device.
However, high-flow electro-osmotic pumps currently for use in microelectronic devices may be constructed using sintered packed-particle porous glass frits as the pumping medium. These glass frits may have a thickness of approximately one to four millimeters, a pore diameter of approximately 1 micrometer, a porosity of approximately 0.2 and a tortuosity of about 1.4. Unfortunately, these pumping medium characteristics may not be ideal for optimizing the pumping action of a high-flow, high-pressure electro-osmotic pump. For example, it may be desirable for the pumping medium to have a pore diameter significantly smaller than 1 micron, and the tortuosity values approximately unity to achieve increased flow rates and pressure per unit area for a given applied voltage. Furthermore, the fabrication of the packed porous oxide frits currently used for electro-osmotic pumps may not be compatible with standard microfabrication processes. These drawbacks may hinder the use of electro-osmotic pumps as effective cooling systems for current and future microprocessors and microsystems.